The present invention relates generally to birefringence in single-mode optical fibers, and in particular, to tuning the level of birefringence in such a fiber for use in, for example, an in-fiber optical isolator.
An in-fiber optical isolator suitable for high power operation is the subject of a co-pending invention (Ser. No. 11/626,095 filed Mar. 8, 2007) and is hereby incorporated by reference. In this isolator, Faraday rotation occurs in a birefringent fused-silica single-mode optical fiber whose polarization beat length has been matched to the period of an array of permanent magnets arranged in a repelling configuration. This Faraday rotation method is called magnetic quasi-phase matching. The beat length is the ratio of the wavelength of light propagating in the core of the fiber to the fiber birefringence. A well-known technique for producing birefringence in the core of an optical fiber is to insert two borosilicate rods into holes drilled into the preform on either side of the core. During the drawing and cooling of the fiber the larger thermal expansion coefficient of the borosilicate results in stress on the core, and this produces birefringence. This type of birefringent fiber in which two circular stress rods are placed on opposite sides of the core is called “panda fiber.” The magnitude of the birefringence produced by this technique, however, cannot be predicted with sufficient accuracy for purposes of the magnetic quasi-phase matching technique. Furthermore, if it is desired to change the operating wavelength, but keep the beat length matched to a fixed magnet array, it is necessary to change the birefringence. Although it is possible to reduce the birefringence by heating a fiber containing stress rods, it is not convenient to do so when the fiber is confined in the gap of a magnet array. Moreover, excessive heating within the magnetic array can cause demagnetization of the array.
It would be desirable to be able to vary the stress in a portion of an optical fiber segment and thereby to vary the birefringence in the entire length of the segment. This would facilitate, for example, matching the beat length to the period of a fixed magnet array to produce Faraday rotation in a high-power optical isolator.